The invention relates to superabsorbent polymer compositions which absorb water, aqueous liquids and blood wherein the superabsorbent polymer compositions of the present invention have improved properties, including high gel bed permeability, fluid retention including achieving higher gel bed permeability without the disadvantages of low retention that are characteristic of higher gel strengths, and compatibility with thermoplastics including hydrophobic thermoplastics and polyolefins. The superabsorbent polymer compositions of the present invention include a coating of a thermoplastic polymer. The present invention also relates to preparation of these superabsorbent polymer compositions and their use as absorbents in hygiene articles and in industrial fields.
Superabsorbent refers to a water-swellable, water-insoluble, organic or inorganic material capable of absorbing at least about 10 times its weight, and up to about 30 times or more its weight in an aqueous solution containing 0.9 weight percent sodium chloride solution in water. A superabsorbent polymer is a crosslinked polymer which is capable of absorbing large amounts of aqueous liquids and body fluids, such as urine or blood, with swelling and the formation of hydrogels, and of retaining them under a certain pressure in accordance with the general definition of superabsorbent. Superabsorbent polymer compositions include post treatment of the particulate polymer including surface crosslinking, surface treatment and post heat treatment. Superabsorbent polymer particles are particles of superabsorbent polymers or superabsorbent polymer compositions. The acronym SAP may be used in place of superabsorbent polymer herein.
The superabsorbent polymer compositions that are currently commercially available are crosslinked polyacrylic acids or crosslinked starch-acrylic acid graft polymers, in which some of the carboxyl groups are neutralized with sodium hydroxide solution or potassium hydroxide solution and surface treated with surface cross linker and optionally other surface additives. As a result of these characteristic properties, these polymers are chiefly used for incorporation into sanitary articles, such as babies' diapers, incontinence products or sanitary towels.
For fit, comfort and aesthetic reasons and from environmental aspects, there is an increasing trend to make sanitary articles smaller and thinner. This is being accomplished by reducing the content of the high volume fluff fiber of these articles. To ensure a constant total retention capacity of body fluids in the sanitary articles, more superabsorbent polymer content is being used in these sanitary articles. As a result of this, superabsorbent polymer compositions must have increased permeability characteristics as well as increased affinity to thermoplastic fibers such as polyolefins, which may be used to replace some or all of the fluff fiber, while retaining other characteristics such as adequate absorption and retention.
Permeability is a measure of the effective connectedness of a porous structure, be it a mat of fiber or a slab of foam or, in this case, crosslinked polymers and may be specified in terms of the void fraction and extent of connectedness of the superabsorbent polymer composition. Gel permeability is a property of the mass of particles as a whole and is related to particle size distribution, particle shape, and the connectedness of the open pores, shear modulus and surface modification of the swollen gel. In practical terms, the permeability of the superabsorbent polymer composition is a measure of how rapidly liquid flows through the mass of swollen particles. Low permeability indicates that liquid cannot flow readily through the superabsorbent polymer composition, which is generally referred to gel blocking, and that any forced flow of liquid (such as a second application of urine during use of the diaper) must take an alternate path (e.g., diaper leakage).
In particular, gel blocking is a well-known problem that may be associated with the use of superabsorbent polymer compositions in absorbent articles such as diapers. Gel blocking occurs when rapid expansion of the superabsorbent polymer particles around the point of entry of body fluid into an absorbent article causes a closing of the interstitial spaces and pores in the SAP-fluff matrix. Since the transport of liquid by diffusion through swollen hydrogel is much slower than transport through the interstitial spaces, a sealing effect occurs in the area of fluid entry. This effect is referred to as gel blocking.
Transportation of liquid through swollen superabsorbent polymer particles themselves follows the laws of diffusion and is a very slow process which plays no role in the distribution of the liquid in the use situation of the sanitary article. In superabsorbent polymer compositions, which cannot maintain an open bed structure to effect capillary transportation because of a lack of gel stability, the separation of the superabsorbent particles from one another has been ensured by embedding the superabsorbent polymer particles into a fiber matrix.
In diaper constructions, for what is called the next generation, there is less fiber material, or potentially none at all, in the absorber layer to assist in transportation of the liquid or maintenance of an open, fluid permeable structure. The superabsorbent polymer composition of these next generation diaper constructions must have a sufficiently high stability in the swollen state, generally called gel strength, so the swollen gel has a sufficient amount of capillary spaces through which liquid can be transported.
To obtain a superabsorbent polymer composition with high gel strength, the degree of crosslinking of the polymer may be increased, which necessarily results in a reduction in the swellability and the retention capacity. To achieve the increased permeabilities needed in extremely thin, next generation articles with low fiber content, current art has taught to increase the amount of crosslinking. However the absorption and retention values of the superabsorbent polymer compositions are reduced to undesirably low levels. It is an important goal of the art of making superabsorbent polymer compositions to develop a composition having a high absorption and retention capacity for liquid in the after-surface crosslinking stage and increased permeability properties. It has been found that by using new surface modifications to the superabsorbent polymer particles, results of higher permeabilities without the undesirable associated low absorption values are achieved.
Superabsorbent polymer compositions are often provided in the form of particulate powders, granules, or fibers that are distributed throughout absorbent fibrous substrate in a core to increase the absorbency of a hygiene article. One problem with the use of superabsorbents is that the superabsorbent composition can be physically dislodged from the fibrous substrate in the core of an absorbent product such as a hygiene article. Separation of the superabsorbent from its fibrous substrate may reduce the absorbency of the hygiene article and may result in superabsorbent polymer composition escaping from the article and diminishes the effectiveness of the superabsorbent polymer composition. In particular it would be advantageous to have the superabsorbent particles affix to polyolefin fibers in a hygiene article in such a fashion as to improve the superabsorbent polymer composition containment and the effectiveness of the hygiene article.
It is therefore an object of the present invention to provide an absorbing polymer composition that exhibits excellent properties such as capabilities of maintaining high liquid permeability and liquid retention even when the superabsorbent polymer is increased in percent by weight based on the absorbent structure and the composition has an acceptable affinity to fibers, especially polyolefin fibers.